This invention relates to a method for converting an oxygenate feedstock to an olefin product. In particular, this invention relates to a method for converting an oxygenate feedstock, including a diluent co-feed, to an olefin product, by contacting the feedstock with a silicoaluminophosphate catalyst at a high total pressure of the feedstock while maintaining a low partial pressure of the oxygenates undergoing reaction.
Olefins, particularly ethylene and propylene, have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Promising alternative feedstocks for making ethylene and propylene are oxygenates. Particularly promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, coke materials, including coal, recycled plastics, municipal wastes, or any appropriate organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for ethylene and propylene production.
In converting oxygenates to ethylene and propylene products, by-products are also formed. Representative by-products include alkanes (methane, ethane, propane, and larger), aromatic compounds, carbon oxides and carbonaceous deposits on and within the catalyst materials (also referred to as xe2x80x9ccokexe2x80x9d).
U.S. Pat. No. 5,126,308, Barger et al., discloses a process for making olefins from methanol. The process incorporates the use of an inert diluent such as helium, argon, nitrogen, carbon monoxide, carbon dioxide, hydrogen, steam, paraffinic hydrocarbons, and aromatic hydrocarbons in the methanol feed. The methanol feed is diluted to more efficiently convert the methanol to olefins, and the process is operated anywhere from a total pressure of 0 psig (101 kPa) to 250 psig (1825 kPa). The amount of diluent used can vary considerably, and usually comprises from about 5 mole percent to about 90 mole percent of the feed. Barger, however, does not disclose compressing the cooled effluent stream in a series of compressors comprising one to four stages with cooling of the compressed products between the stages.
U.S. Pat. No. 5,811,621, van Dijk, discloses a process for recovering ethylene from a product stream produced by a methanol to olefin reaction. The reaction is carried out at a pressure of from about 1 to about 20 atmospheres. Higher methanol partial pressures are desired to enhance the production of propylene in the product stream, while lower methanol partial pressures are desired to enhance ethylene production. However, Van Dijk suffers from the same deficiencies as that of Barger.
In converting oxygenate-containing feedstock to ethylene and propylene product, while better conversion to olefin product is desired, an improved product recovery scheme is also desired.
This invention relates to an improved process for converting oxygenates to light olefins comprising operating an oxygenate conversion reactor at high pressures to produce a reactor effluent having a pressure that provides minimal compression requirements required to operate an olefin recovery system.
One aspect of the present invention is directed to a method for making an olefin product from an oxygenate-containing feedstock including an oxygenate and a diluent. The method comprises the following steps: contacting, in a reactor, an oxygenate-containing feedstock, including an oxygenate and a diluent, with a silicoaluminophosphate molecular sieve under conditions effective to form a reactor effluent; cooling at least a portion of the reactor effluent to form a cooled olefin product stream comprising ethylene and propylene at a pressure of at least about 30 psia (307 kPa) and a liquid diluent stream including at least a portion of the diluent contained in the oxygenate containing feedstock and compressing at least a portion of the cooled olefin product stream to a pressure of about 165 psia (1138 kPa) to about 600 psia (4137 kPa).
Desirably, in this method, the total pressure of the oxygenate-containing feedstock at an entrance of the reactor is about 40 psia (276 kPa) to about 600 psia (4137 kPa), the partial pressure of the oxygenate at the entrance of the reactor is about 15 psia (103 kPa) to about 150 psia (1034 kPa) and the partial pressure of diluent is about 1 psia (7 kPa) to about 585 psia (4033 kPa).
Another aspect of the present invention is directed to a method for making an olefin product from an oxygenate-containing feedstock including an oxygenate and a diluent. The method comprises the following steps: contacting, in a reactor, an oxygenate-containing feedstock, including an oxygenate and a diluent, with a silicoaluminophosphate molecular sieve under conditions effective to form a reactor effluent; cooling at least a portion of the reactor effluent to form a cooled olefin product stream including ethylene and propylene; and introducing at least a portion of the cooled olefin product stream directly to an olefin recovery system at a pressure of about 165 psia (1138 kPa) to about 600 psia (4137 kPa).
Desirably, in this aspect of the invention, the total pressure of the oxygenate-containing feedstock at an entrance of the reactor is about 180 to about 600 psia (4137 kPa), the partial pressure of the oxygenate at the entrance of the reactor is about 15 psia (103 kPa) to about 150 psia (1034 kPa) and the partial pressure of diluent is about 30 psia (207 kPa) to about 585 psia (4033 kPa).
The present invention will be better understood by reference to the following description of the invention when taken together with the appended claims and attached drawings.